Aluminum alloys are well recognized for their corrosion resistance. In the automotive industry, aluminum alloys are used extensively for tubing because of their extrudability as well as the combination of light weight and high strength. They are used particularly for heat exchanger or air conditioning condenser applications, where excellent strength, corrosion resistance and extrudability are necessary.
The AA1000 series aluminum alloys are often selected where corrosion resistance is needed. Where higher strengths are required, the AA3000 series aluminum alloys are often used.
A difficulty with use of aluminum alloys in corrosive environments, such as heat exchanger tubing, is pitting corrosion. There are various methods currently in use for protecting tubing manufactured from clad sheet stock from corrosion. One of these consists of using a core alloy e.g. X800 or X900, which protects from corrosion through the formation of a dense precipitate band within the core adjacent to the cladding. This band corrodes preferentially to the rest of the core thereby increasing the life of the tube. This method is only applicable to sheet products due to the need for a clad layer of Al—Si alloy on the tube surface.
Alloys currently in use for brazed extruded tubing do not give good corrosion resistance unless the tubing is sprayed with zinc prior to brazing. This is typically carried out in-line at the extrusion press and is costly and undesirable from an environmental viewpoint.
Sircar, WO 97/46726, published Dec. 11, 1997 describes a corrosion resistant AA3000 series aluminum alloy containing low copper, but high zinc and titanium. While this alloy provides reduced corrosion, its titanium content of 0.03 to 0.30% by weight raises the pressures required for extrusion, thereby lowering productivity.
U.S. Pat. No. 5,286,316 (Wade) describes an alloy consisting essentially of 0.1-0.29% by weight manganese, 0.05-0.12% by weight silicon, 0.10-0.20% by weight titanium, 0.15-0.25% by weight iron with the balance aluminum. This alloy contained very low levels of copper not exceeding 0.03% by weight, but again a quite high titanium content.
In Auran et al, EP 0899350, published Mar. 3, 1999, a corrosion resistant alloy is described containing 0.10-0.40% by weight iron, 0.05-0.25% by weight silicon, 0.12-0.22% by weight titanium, less than 0.10% by weight manganese, less than 0.35% by weight copper and the balance aluminum.
In Auran et all WO 99/04051, published Jan. 28, 1999, another corrosion resistant alloy is described preferably consisting of 0.06-0.15% by weight iron, 0.05-0.15% by weight silicon, 0.03-0.08% by weight manganese, 0.10-0.18% by weight titanium, 0.10-0.18% by weight chromium, less than 0.01% by weight copper, 0.10-0.18% by weight zinc and the balance aluminum.
Jeffrey et al., CA 2,247,037 teaches extruding aluminum alloys having generally high zinc and low titanium into heat exchanges tubing. Corrosion testing was conducted on tubing which had not been exposed to furnace brazing conditions. An alloy with a low zinc content (0.0022%) failed the corrosion test.
Other references include Hufnagel W., “Key to Aluminum Alloys, 4th Edition” 1992 showing typical compositions for 1000 and 3000 series aluminum alloys.
U.S. Pat. No. 5,976,278 describes an aluminum alloy containing up to 0.03% copper, 0.1-1.5% manganese, 0.03-0.35% titanium, up to 1.0% magnesium, up to 0.01% nickel, 0.06-1.0% zinc, up to 0.3% zirconium, up to 0.50% iron and silicon and up to 0.20% chromium.
An aluminum alloy is described in U.S. Pat. No. 5,906,689 containing up to 0.03% copper, 0.1-0.5% manganese, 0.03-0.30% titanium, 0.06-1.0% zinc, up to 0.50% iron, 0.05-0.12% silicon, up to 0.01% manganese, up to 0.01% nickel and up to 0.5% chromium.
In U.S. Pat. No. 5,286,316, an aluminum alloy is described containing 0.1-0.5% manganese, 0.05-0.12% silicon, 0.10-0.20% titanium and 0.15-0.25% iron.
JP-A-2000-119784 describes a further aluminum alloy containing manganese along with magnesium, copper, silicon, zinc, nickel and iron.
It is an object of this invention to produce an aluminum alloy of the AA3000 type having excellent corrosion resistance when extruded into tubing and brazed without the need to add zinc by either zinc spraying or by alloy addition.